Sega Genesis

The Sega Megadrive, or Sega Genesis as it was known in North America, was Sega’s 16 bit console. Designed around a Motoral 68000 and Zilog Z80 pairing, which was very common at the time, the console had an elegant yet straightforward architecture which made it very easy for programmers to exploit its power.

SMS Mode

The Sega Genesis is natively backwards compatible with the Sega Master System. In fact, most of the hardware contained in the Master System is also found in a Genesis: Z80 processor, PSG, etc…

To enter Master System compatibility mode, it seems that you must, at the very least, do the following (refer to the Pinouts section for Genesis pin locations):

Genesis Pin

Name

Connect to

B30

#M3

GND

B9

A21

Vcc

B10

A22

GND

With the above connections, the cartridge port loses the #RESET signal on B27 – it now constantly outputs GND instead of a nice active-low reset pulse when the reset button is pressed. Therefore, you cannot rely on B27 for reset when in Master System compatibility mode. This is the source of much frustration for me! The cartridge can still send a reset signal to the console via pin B2 in Master System mode.

Pin Reconfiguration

Other Genesis pins change functions when entering Master System compatibility mode. There is very little information available on these, the information listed below was found on SMSPower.org and through experimentation. The list is not exhaustive and I will keep adding to it as I discover more…

Genesis Pin

Genesis Name

SMS Function

B28

#LDSW

#WR

B17

#CE

#M0-7

B16

#OE

#RD

B11

A23

#PAUSE

B6

A18

#CE

B7

A19

#IORQ

B27

#RESET

GND

Power

Console Current Consumption

These values were measured on NTSC consoles using a 9V benchtop power supply. These values are averages because the actual current consumption is dynamic – though according to my experiments they don’t appear to vary much.

Unit

Avg (mA)

Peak (mA)

Notes

Model 1 HD Graphics

700

700

Measured with and without a game inserted (Sonic 2), didn’t seem to make a difference.

Model 1 Sega CD

500

1000

Peak occurs right after booting. The Sega CD does not power up if a Genesis cartridge is inserted.

Model 2 Sega CD

500

1000

Peak occurs right after booting. The Sega CD does not power up if a Genesis cartridge is inserted.

Model 2 Genesis

300

300

Measured with and without a game inserted (Sonic 2), didn’t seem to make a difference.

32X with 32X game

350

350

Measured with Doom

32X with Genesis game

200

200

Measured with Sonic 2

Game Cart Current Consumption

These values were measured on an NTSC Model 2 Genesis. These values are averages because the actual current consumption of cartridges is dynamic.